Submicron structuring in advanced semiconductor processes achieves a high integration density. Submicron structuring by means of advanced lithographic techniques permits fabrication of greatly scaled down CMOS transistors. The achievable bandwidth and the available driver capacity is, however, restricted in CMOS processes. It is particularly for analog circuits requiring a high cutoff frequency that bipolar transistors are preferably employed, fabricated, for example, from gallium arsenide. Gallium arsenide transistors do not permit integration, however, in conventional CMOS processes. This is why more recently process technologies have been developed comprising fast bipolar transistors combinable with advanced CMOS technologies. One of these technologies is the SiGe—BiCMOS technology in which the base connections of the bipolar transistors are formed by a layer of silicon-germanium (SiGe). By injecting atoms of germanium into the base layers of the SiGe transistors the band gap energy is reduced so that these transistors feature substantially higher cutoff frequencies than transistors fabricated by a conventional bipolar method or BiCMOS method.
Injecting atoms of germanium into the base layers of bipolar transistors is the key to enhanced performance of these transistors. The silicon-germanium base layer is usually deposited by epitaxy.
Complementary transistors are often used in circuitry for implementing specific analog functions. Simultaneous epitaxial growth of the base layers of both transistors (NPN and PNP) does not permit optimizing the germanium profile, however. For as high a cutoff frequency as possible PNP transistors and NPN transistors require differing germanium profiles.